posix: missing headers for Linux portability.

* ext/posix/pwd.c: Include unistd.h for geteuid and others.
* ext/posix/stdlib.c: Include fcntl.h for open.
* ext/posix/sched.c: Include unistd.h for
_POSIX_PRIORITY_SCHEDULING.
* ext/posix/sys/msg.c, ext/posix/sys/socket.c,
ext/posix/syslog.c: Include unistd.h for _POSIX_VERSION.
* ext/posix/sys/times.h: Include unistd.h for sysconf.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>